Correlation of Real-time CO2 Consumption to Biomass and Lipid Production under Silicon and

Nitrate Limited Photobioreactor Cultivation of the Diatom Cyclotella

Altan Ozkan, Omar Chiriboga, and Gregory L. RorrerSchool of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, OR 97331In algal photobioreactors (PBRs), productivity of biomass and its metabolites of commercial interest depend upon the delivery of inorganic carbon to the culture suspension and its consumption by the algal cells. Diatom algae are unique in that dissolved silicon is a required substrate for cell division. Consequently, silicon delivery to diatoms is unique approach for controlling cell division that is independent of other macronutrients. To study the correlation of carbon dioxide (CO2) delivery and consumption to the production of biomass and lipids during cultivation of photosynthetic diatom Cyclotella under various regimes of dissolved silicon and nitrogen limitation, a bubble-column photobioreactor was instrumented to provide controlled CO2 concentration in the inlet aeration gas and real-time measurement of CO2 concentration in the outlet gas. During photobioreactor cultivation, the cell suspension was assayed for cell number density, dissolved nitrate concentration, and silicon concentration, and biomass samples as well as lipids extracted were further analyzed for C/N content. Real time measurement of CO2 consumption successfully predicted biomass productivity during cultivation and was used to study two regimes of nutrient limitation. In the first regime, the culture was grown to silicon limitation with availability of nitrate (Si-limitation). In the second regime, the culture was grown to silicon limitation and then to nitrogen limitation (Si, N co- limitation). In all cultivation experiments, after silicon limitation and stationary phase of growth was achieved, the cell number density was constant but inorganic carbon consumption per cell continued, which was correlated to lipid production. Under Si limited cultivation, the inorganic carbon consumption was about 50% larger after 8 days in stationary phase relative to Si, N-co- limited cultivation alone, and lipid content approached 50 wt%. Under Si-limited growth, the overall biomass productivity was higher but the lipid content was lower, and so when the biomass productivities were taken into the account, the lipid productivities from both Si-limited and Si, N co-limited cultivation modes were similar. Moreover, while in log phase about 15% of carbon consumed were used for lipid production in both reactors, during the stationary phase this ratio goes up to about 60 and 88% under Si-limitation, and Si, N co-limitation regimes, respectively. The PBR system and results presented here demonstrate the unique capability of real-time CO2 consumption measurement for metabolic assessment of biomass and lipid productivity in photosynthetic diatom cultures.

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